1. Field of the Invention
The present invention relates to a diversity reception technique.
2. Description of the Related Art
In radio communication, it is known that a multipath environment is established in which a plurality of elementary waves that have propagated via various propagation paths arrive at a reception apparatus at different timings, and transmission error may occur due to distortion of received waveforms in some locations. In such a multipath environment, space diversity is widely known as a technique to improve communication quality. In space diversity, simultaneous communications are performed by spatially distancing a plurality of antennas from one another so as to take advantage of the fact that communication quality significantly fluctuates depending on positions of antennas. In this way, for example, radio signals received via the plurality of antennas have different levels of communication quality, thereby increasing the probability of achieving sufficient reception quality via at least one of the antennas. It should be noted that a reception apparatus can obtain reception data from a plurality of signals received via a plurality of antennas, for example, by demodulating the received signals through application of selection combining or maximum ratio combining to the received signals, or by performing maximum likelihood estimation based on the received signals.
It is thought that the aforementioned diversity effects can further be enhanced by a spatially-wide distribution of a plurality of antennas (diversity branches). In this case, the plurality of antennas that have been distributed over a wide area are spatially distanced from a control function unit that executes reception processing such as selection combining, and therefore the control function unit is wiredly connected to the plurality of antennas via cables and the like. Signals received via the plurality of antennas are transmitted to the control function unit via wired transmission paths, and the control function unit applies reception processing to the received signals.
It should be mentioned that, in the case where the control function unit is wiredly connected to the plurality of antennas, the control function unit needs to include interfaces for wired communication that are equal to or larger than the plurality of antennas in number. In order not to affect the communication speed in an entire communication system utilizing both radio sections and wired sections, these interfaces for wired communication are required to perform communication at a speed higher than the speed in the radio sections. For example, in a radio communication system where an uncompressed high-definition video is transmitted from a transmitter by radio and received by a site-diversity radio reception station, the control function unit needs to include a plurality of interfaces for wired communication of over 1 Gbps. In terms of hardware complexity and cost, it is not easy to thus configure the control function unit to include a larger number of interfaces for high-speed wired communication. Furthermore, as the number of antennas cannot be increased to the point where it surpasses the number of the interfaces for wired communication included in the control function unit, extensibility and flexibility of the system are restricted.
On the other hand, in the case where a certain diversity branch is positioned far away from the control function unit, it is required to lay a long communication cable. In terms of cost or availability, it is not always easy to use a long communication cable. There is also a problem that, due to concentration of a large number of communication cables around the control function unit, wiring of cables becomes complicated.
In contrast, there are studies on a daisy-chained diversity reception station in which a plurality of diversity branches (communication units provided with antennas) are connected in serial by communication cables (see Japanese Patent Laid-Open No. 2005-533405 and No. 2004-056346). More specifically, the communication units provided with the antennas are wiredly connected in serial, and each communication unit, for example, combines a signal received via its antenna and a signal received via the antenna of a lower-order unit in the daisy chain, and transfers the combined signals to a higher-order unit. In such case where the communication units provided with their respective antennas are connected in a daisy chain, a control function unit need not include interfaces for wired communication that are equal to the antennas in number. Herein, from a standpoint of one communication unit, a communication unit closer to the control function unit in the daisy chain is referred to as a higher-order unit, and a communication unit farther from the control function unit in the daisy chain is referred to as a lower-order unit. Similarly, from a standpoint of one communication unit, a direction toward the control function unit is referred to as a higher-order direction, and the opposite direction is referred to as a lower-order direction.
Japanese Patent Laid-Open No. 2005-533405 describes a technique in which communication units generate and output a new combined signal and combined confidence index from the following four types of input: a received radio signal, a confidence index thereof, a combined signal received from a lower-order unit in a daisy chain, and a combined confidence index thereof. All of the communication units connected in the daisy chain execute this operation; as a result, the control function unit can obtain weighted and combined received signals from a highest-order communication unit.
Meanwhile, Japanese Patent Laid-Open No. 2004-056346 describes a technique in which a communication unit compares a reception level of a radio signal it has received with a reception level of a radio signal that has been received by and input from a lower-order unit, and accordingly selects a signal to be transmitted to a higher-order unit. More specifically, according to Japanese Patent Laid-Open No. 2004-056346, a communication unit transmits a radio signal it has received together with a reception level thereof to a higher-order unit if a reception level of the radio signal it has received is higher than a reception level of a radio signal received by a lower-order unit. On the other hand, a communication unit relays a radio signal that has been received by and input from a lower-order unit together with a reception level thereof to a higher-order unit if a reception level of a radio signal it has received is equal to or lower than a reception level of the radio signal received by the lower-order unit. This operation is repeated in order from a lowest-order communication unit to a highest-order communication unit; as a result, the control function unit can obtain, from the highest-order communication unit, a signal that has been received by a communication unit with the highest reception level.
However, in order to make meaningful use of the technique described in Japanese Patent Laid-Open No. 2005-533405, a combined signal transmitted between communication units must have a soft-decision value, which gives rise to the problem that an interface for wired communication needs to have a high communication performance. For example, in order to transmit a soft-decision value quantized using 1 byte as a combined signal, an interface for wired communication needs a communication speed eight times faster than a communication speed in radio sections. On the other hand, with the technique described in Japanese Patent Laid-Open No. 2004-056346, a communication unit has to convey a reception level of a radio signal to a higher-order unit, which gives rise to the problem of an increase in the scale of the communication unit as an apparatus, as well as an increase in the amount of information to be transmitted.
The present invention has been made in view of the above problems, and enables achievement of space diversity effects with a simpler apparatus configuration.